If you check it frame-by-frame in pI, it's correct -- there's no difference between rendering and preview.
I suspect it may be a framerate issue. The easiest way to avoid this: don't output AVI. Just output an image sequence.
Also, the feature you're looking for is called "replace", and *is* covered in the docs. Just r-click on the stage emitter and select "replace".
Alan.
....................
Alan Lorence
Product Manager
GenArts, Inc.
http://www.genarts.com
http://www.wondertouch.com